Cutting machines are typically used to accurately produce large machined parts having complex shapes from generally planar, metal workpieces. It is desirable to automate the cutting process to increase productivity and quality while decreasing manufacturing and labor costs. A particular method of automating the cutting process, namely numerical control of the cutter, has been applied to cutting machines since at least the late 1950's. Numerical control involves the use of a programmed controller to control the motion of the cutter with numeric commands, and thus produce a part having a predetermined size and shape. With the availability of microcomputers and interactive software, modern numerical control cutting machines typically employ computer numerical control ("CNC") systems.
The use of CNC to automate the cutting process requires the controller to locate the edges and determine the orientation of the workpiece relative to the orthogonal axes of the cutting machine. Locating the edges and determining the orientation of the workpiece is typically accomplished by manually positioning a digitizing probe at successive locations around the perimeter of the workpiece. The digitizing probe provides the controller with the orthogonal coordinates of the digitized locations so that the software of the controller can map the perimeter, and thus the footprint of the workpiece relative to the orthogonal axes of the cutting machine. Locating the edges and determining the orientation of the workpiece is necessary to verify that a particular part having a predetermined size and shape can be produced from the workpiece, and to reduce the amount of scrap that results from machining the part.
Conventional CNC cutting machines require an operator to manually position the digitizing probe above at least three corners of a square or rectangular shaped workpiece. From the digitized information, the controller maps the footprint of the workpiece and determines the optimum path for the cutter to produce the desired part. Manual digitizing on a conventional CNC cutting machine, however, is time consuming and is limited to workpieces that are generally square or rectangular shaped unless additional locations on the perimeter of the workpiece are digitized. Accordingly, the unused portions of prior workpieces, known in the cutting machine art as scraps or remnants, that are not generally square or rectangular shaped cannot easily be used to produce additional parts.
Automated methods of locating the edges and determining the orientation of a workpiece relative to the orthogonal axes of a CNC cutting machine include the use of sensors known in the art as line followers, or line trackers. For example, U.S. Pat. No. 4,518,856, issued May 21, 1985 to Blackington discloses an optical thin line-tracing sensor. The head of the sensor includes a source optic fiber for transmitting a light beam in the direction of a sheet of material having a line drawn, printed or otherwise affixed to the material. The material surrounding the line has a contrasting energy-reflective property for providing electrical signals representative of the movement of the sensor relative to the line.
The use of the optical thin line-tracing sensor disclosed in the patent to Blackington to locate the edges and determine the orientation of a workpiece relative to the orthogonal axes of a CNC machine creates a significant problem. Optical line-tracking sensors rely on the optical contrast between the line being followed and the material of the background. Typically, the supporting table of a CNC cutting machine is made of metal and includes a horizontal grate consisting of longitudinal and lateral rails. Thus, there is little optical contrast between the edges of the workpiece and the supporting table even when an edge of the workpiece is adjacent a rail of the supporting table. The perimeter of the workpiece may be painted to provide optical contrast between the edges of the workpiece and the background. Painting the perimeter of the workpiece, however, is time consuming and accordingly diminishes the advantages achieved by automating the cutting process.
It is possible to use a capacitive proximity sensor to locate the edges and determine the orientation of a workpiece relative to the orthogonal axes of a CNC machine. A line follower that measures capacitance, however, likewise creates a significant problem. The responsiveness of a capacitive proximity sensor to most common metals is insufficient without amplification to provide the precision necessary to accurately locate the edges and determine the orientation of a workpiece relative to the orthogonal axes of the CNC cutting machine.
Regardless of the type of sensor used to map the perimeter of the workpiece, the output from the sensor is typically either a digital or an analog electrical signal. The electrical signal from a digital sensor does not require conversion (from an analog signal) for further processing by the controller. A digital sensor, however, indicates whether any portion of the sensor is above the workpiece. Thus, the electrical signal from a digital sensor is an "on" or an "off" indication and does not indicate how much of the sensor is above the workpiece. Consequently, a digital sensor is not as precise as an analog sensor, and the digitizing probe tracks the perimeter of the workpiece with pronounced zig-zag movements. To achieve the same precision and smooth movement obtained from a predetermined array of analog sensors would require significantly more, or significantly smaller digital sensors.
Accordingly, those skilled in the art of CNC cutting machines have found it difficult to automatically locate the edges and determine the orientation of a generally planar, metal workpiece relative to the orthogonal axes of the cutting machine. In particular, those skilled in the art of CNC cutting machines have found it difficult, if not impossible, to utilize remnant workpieces that are not generally square or rectangular shaped, or that include internal cutouts, to accurately produce complex machined parts having a predetermined size and shape. As will be made apparent by the following description, the digitizing probe and method of the invention solves these and other problems.